Thin film solar cell absorbers composed of earth-abundant elements such as Cu2ZnSn(S,Se)4 (CZT(S,Se)) are particularly relevant due to their relatively low toxicity and their record maximum power conversion efficiency of 12.6%. Despite promising results, further work is however needed to understand how to improve this technology and enable its commercial-scale implementation.
Recent efforts have identified the need to remove defect states by deliberate passivation or the introduction of dopants in order to improve upon the voltage deficits (compared to theoretical limits) exhibited by CZT(S,Se). The inclusion of silver could reduce tail states that are introduced by the disorder caused in the random alternation of copper and zinc in the kesterite lattice (the copper cation is only 5% larger than the zinc cation). The inclusion of silver has been shown to reduce antisite defects in CZT(S,Se) absorbers by approximately an order of magnitude.
One process to incorporate silver in CZT(S,Se) involves evaporating silver after the deposition of the absorber. This technique can however compromise absorber quality by exposing it to air.
An alternative method involves nanoparticle synthesis. See, for example, Wei et al., “Synthesis and Characterization of Nanostructured Stannite Cu2ZnSnSe4 and Ag2ZnSnSe4 for Thermoelectric Applications,” ACS Appl. Mater. Interfaces, April 2015, 7, 9752-9757. However, this synthesis procedure requires extensive processing and multiple purification steps which in turn lead to extremely low yields of nanoparticles. Further, the native ligands that surround the particles are insulating and thus yield low quality devices.
Accordingly, improved techniques for incorporating silver into absorber materials like CZT(S,Se) would be desirable.